Echo Modeling Method for Shipborne Coherent Microwave Oceanographic Radar

Abstract

This invention proposes an echo modeling method for a shipborne coherent microwave ocean radar, comprising: performing three-dimensional sea surface modeling to simulate the three-dimensional sea surface; simulating the instantaneous state and evolution of breaking waves based on the conditions for the generation of breaking waves on the simulated three-dimensional sea surface; simulating a ship motion model based on a three-harmonic statistical model of periodic motion; and simulating ship motion models for different sea conditions by varying the amplitude and angular frequency of each harmonic of the ship motion model to simulate the ship's swaying state under different sea conditions; simulating a composite sea surface echo velocity model modulated by the ship's motion; and using small-amplitude scattering theory to model the scattering mechanism of free waves and breaking waves by a coherent microwave ocean radar at a ship's grazing angle, thereby obtaining a simulation model of the Doppler spectrum of the coherent microwave ocean radar's sea surface echo. This invention establishes the scattering mechanism of the coherent microwave radar on a ship, taking into account the influence of the ship platform and breaking waves on the radar's Doppler spectrum.

Description

Technical Field

[0001] The invention belongs to the field of microwave radar ocean remote sensing, and in particular relates to an echo modeling method of a shipborne coherent microwave ocean radar. Background Art

[0002] Wave parameters, such as wave height, period, and direction, are crucial for disaster prediction, marine engineering, and oceanographic research. Shore-based coherent microwave radars can accurately measure wave parameters with high temporal and spatial resolution. On the other hand, with the development of electronic technology, shipborne radars are able to achieve greater wave observations. However, the scattering mechanism of shipborne coherent microwave radars has not yet been established. There is still limited research on the characteristics of sea surface echoes from shipborne coherent microwave radars. Only a few works briefly discuss the impact of a single factor in ship motion on the Doppler spectrum of radar sea surface echoes. For example, the forward speed of the ship and the angle between the radar illumination direction and the bow of the ship affect the radar backscattering.

[0003] In summary, existing technologies do not systematically explain the scattering mechanism of shipborne coherent microwave ocean radars and perform echo modeling and simulation. In order to improve the wave measurement performance of shipborne coherent microwave radars and promote their widespread application, it is necessary to propose an echo modeling method for shipborne coherent microwave radars. Summary of the Invention

[0004] In order to solve the problem that the existing technology cannot realize shipborne coherent microwave radar echo simulation modeling, the present invention establishes a shipborne coherent microwave ocean radar echo modeling method that takes into account factors such as the forward motion of the ship, the six-degree-of-freedom shaking of the ship, the angle between the radar illumination direction and the bow direction, and breaking waves.

[0005] According to one aspect of an embodiment of the present invention, a method for modeling echoes of a coherent microwave ocean radar with ship platform breaking coupling is provided, comprising: performing three-dimensional sea surface modeling to simulate the three-dimensional sea surface; simulating the instantaneous state and evolution process of the breaking waves based on the conditions for the generation of breaking waves on the simulated three-dimensional sea surface; simulating a ship motion model of a third harmonic statistical model based on periodic motion, and simulating ship motion models of different sea conditions by changing the amplitude and angular frequency of each harmonic of the shaking of the ship motion model to simulate the swaying state of the ship in different sea conditions; simulating a composite sea surface echo velocity model modulated by the ship motion; and using small amplitude scattering theory to model the scattering mechanism of the coherent microwave ocean radar through free waves and breaking waves at a ship grazing angle to obtain a coherent microwave ocean radar sea surface echo Doppler spectrum simulation model.

[0006] The present invention establishes an echo modeling method for a ship-borne coherent microwave oceanographic radar with fragmentation coupling. Its purpose and advantage lies in addressing the existing lack of a systematic explanation of the scattering mechanism of shipborne coherent microwave radars and their echo modeling simulation. This method, therefore, takes into account factors such as the ship's forward motion, the ship's six-degree-of-freedom roll, the angle between the radar illumination direction and the ship's bow, and breaking waves. This method is beneficial for improving the wave measurement performance of shipborne coherent microwave radars and promoting their widespread application. BRIEF DESCRIPTION OF THE DRAWINGS

[0007] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings of the embodiments are briefly introduced below.

[0008] Figure 1 This is an overall technical block diagram of an echo modeling method for a shipborne coherent microwave ocean radar provided by one embodiment of the present invention.

[0009] Figure 2 The sway angles for simulating high, medium and low sea conditions are provided by an embodiment of the present invention.

[0010] Figure 3 A schematic diagram of the six-degree-of-freedom motion of a ship platform provided in one embodiment of the present invention.

[0011] Figure 4 This is a schematic diagram of a scattering model of free waves and breaking waves coupled under a ship platform provided by one embodiment of the present invention.

[0012] Figure 5 This is a comparison diagram of simulation and actual measurement of the range Doppler spectrum and average Doppler spectrum as the angle between the radar line of sight and the main wave direction changes according to one embodiment of the present invention.

[0013] Figure 6 This is a comparison diagram between simulation and measurement of the average spectral width of the average Doppler spectrum provided by one embodiment of the present invention as the angle between the radar line of sight and the main wave direction changes.

[0014] Figure 7 This is a comparison diagram of the simulated and measured range Doppler spectrum and average Doppler spectrum provided by one embodiment of the present invention at different ship speeds and different angles between the radar line of sight and the ship's bow.

[0015] Figure 8 This is a comparison diagram between simulation and measurement of the average spectral width of the average Doppler spectrum provided by one embodiment of the present invention as the angle between the radar line of sight and the ship's bow changes.

[0016] Figure 9 This is a comparison diagram of the simulated and measured wavenumber-frequency spectrum of the average Doppler spectrum provided by one embodiment of the present invention. DETAILED DESCRIPTION

[0017] Figure 1 The figure shows the overall technical block diagram of an echo modeling method for a shipborne coherent microwave ocean radar. Figure 1 This method includes: 3D sea surface modeling; determining the conditions for breaking waves on the 3D sea surface; modeling the ship's forward motion and simulating its six-degree-of-freedom motion under various sea conditions; establishing a composite sea surface echo velocity model beneath the ship platform; modeling the microwave scattering mechanism at near-grazing angles from the ship platform to derive a coherent microwave radar sea surface echo Doppler spectrum simulation model; and verifying the accuracy of the simulation model by comparing it with the measured Doppler spectrum of a ship-borne coherent microwave radar. This method is described in detail below.

[0018] 1) Three-dimensional sea surface modeling.

[0019] Combining the JONSWAP undirected wave spectrum and the Longuet-Higgins directional distribution function, a three-dimensional sea surface model is constructed based on linear wave theory. The JONSWAP spectrum model is expressed as:

[0020]

[0021] Where,

[0022] ω p is the peak angular frequency of the wave spectrum, U 10 is the average wind speed at 10m above sea level, F e is the length of the wind zone;

[0023] α is the scale coefficient;

[0024] is the peak shape parameter, whose value is related to the peak frequency;

[0025] γ is the peak enhancement factor.

[0026] The Longuet-Higgins directional distribution function is a non-negative function with an interval of (0, 2π) and an integral value of 1. It is the probability distribution of the direction of the wave. The model is expressed as:

[0027]

[0028] Where,

[0029] F g (s) is the normalization factor;

[0030] s is the expansion factor;

[0031] α m is the wind direction angle, corresponding to the main wave direction.

[0032] Linear ocean wave theory is a model that describes the fluctuations of the ocean surface. It can provide a reasonable description of the numerical model of the ocean surface and the water-air exchange interface. It describes the displacement of the ocean wave surface as the superposition of a series of sinusoidal signals:

[0033]

[0034] Where,

[0035] h(x,y,t) is the three-dimensional sea surface;

[0036] (x, y) is the spatial position on the sea surface; t is the time;

[0037] j=1,2,...M is the azimuth number; i=1,2...N is the frequency number;

[0038] a ij is the amplitude; k i is the wave number;

[0039] φ j is the angle between the sinusoidal signal and the main wave direction;

[0040] ε ij is a random phase, uniformly distributed from -π to π.

[0041] By changing the peak angular frequency U of the wave spectrum 10 and F e To simulate different sea conditions.

[0042] 2) Determine the occurrence conditions of simulated three-dimensional sea surface breaking waves and simulate the instantaneous state and evolution process of breaking waves.

[0043] The conditions for the occurrence of the simulated three-dimensional sea surface breaking waves are determined in step 2. The critical value is that the breaking waves are generated when the wave displacement slope is less than -tan7°. The phase velocity v of the mesoscale wave b Calculated as:

[0044]

[0045] Where c p is the phase velocity of the main wave. The duration of the evolution process is about 0.8 mesoscale wave period.

[0046] 3) The ship's motion model is appropriately simplified without distortion, and a ship motion model based on the third harmonic statistical model of periodic motion is simulated.

[0047] The calculation formula for the sway angle of the ship motion model of the third harmonic statistical model based on periodic motion described in step 3 is:

[0048]

[0049] Long distance:

[0050]

[0051] Bow remote:

[0052] φ Y =A Yi sin(ω Yi t)

[0053] Where,

[0054] A Ri 、A Pi 、A Yi are the amplitudes of the harmonics of roll, pitch and yaw respectively;

[0055] ω Ri 、ω Pi 、ω Yi are the angular frequencies of the harmonics of roll, pitch and yaw respectively.

[0056] The ship motion model for high, medium and low sea conditions is simulated by changing the amplitude and angular frequency of each harmonic of the rocking. For the specific rocking angle simulation, see Figure 2 For the six-degree-of-freedom motion of the ship platform, see Figure 3 .

[0057] 4) Simulation of the composite sea surface echo velocity model modulated by ship motion.

[0058] The composite sea surface echo velocity model modulated by the ship motion in step 4 is the velocity contribution caused by the forward motion and attitude of the ship, and is expressed by the radial velocity of the free wave backscattering as:

[0059] v Rf =V P +v ro ±v f ·cosε

[0060] The radial velocity backscattered by the breaking wave is expressed as:

[0061]

[0062] Where,

[0063] v f is the phase velocity of the free wave;

[0064] v s ∈[v f ,v b cosφ m ] represents the speed of the breaking wave during the breaking evolution process, that is, v sFirst, it is equal to the mesoscale wave phase velocity v b and the azimuth angle φ between the radar line of sight and the main wave direction m The product of , eventually reduced to the free wave phase velocity v f ;

[0065] The positive (negative) sign indicates that the Bragg wave is moving toward (away from) the antenna;

[0066] v ro is the radial orbital velocity of the long wave, which is the main periodic modulation term of the Doppler velocity;

[0067] ε is the radar grazing angle;

[0068] V P is the radial velocity caused by the ship's forward motion and attitude, which can be expressed as

[0069]

[0070] Where,

[0071] It is the angle between the radar line of sight and the ship's heading;

[0072] (v x ,v y ,v z ) are the velocity components in the three directions of the coordinate system generated by the ship's motion and attitude;

[0073] (v R ,v P ,v Y ) represents three linear velocities: horizontal linear velocity v R , longitudinal linear velocity v P and bow telescopic linear speed v Y The model is calculated by combining the position of the antenna relative to the center of gravity of the ship and the swing angle of the ship motion model in step 3. (v x ,v y ,v z ) can be modeled using the following conversion formula:

[0074]

[0075] Where,

[0076] v Surge ,v Sway and v Heave are rolling speed, pitching speed, and heave speed respectively; v F Indicates the forward speed of the ship.

[0077] 5) Using the small amplitude scattering theory, the scattering mechanism of microwave radar through free waves and breaking waves at the grazing angle of the ship platform is modeled, and a coherent microwave radar sea surface echo Doppler spectrum simulation model is obtained.

[0078] The microwave radar at the ship platform grazing angle described in step 5 is modeled using the scattering mechanism of free waves and breaking waves, while taking into account the evolution of breaking waves and the motion of the ship platform. Its Doppler spectrum is expressed as:

[0079]

[0080] In the formula The Doppler spectrum obtained by free wave scattering

[0081]

[0082] The Doppler spectrum obtained by breaking wave scattering

[0083]

[0084] In the above two formulas,

[0085] r is the radial distance; t is the time; is the beam width; k0 is the electromagnetic wave number; f is the Doppler frequency; g is the p is the geometric factor of vertical polarization; ψ is the short-wave spectral density; φ represents the long-wave tilt angle of the incident plane, and P(φ) is the probability distribution of the long-wave slope. f (f,δ) and F s The shape of the Doppler spectrum (f,δ) follows a Gaussian distribution:

[0086]

[0087] Where j is f or s; δf f The width of the Doppler spectrum is caused by the free wave, and δf s It is determined by the free wave and the limited range of the breaking wave. The Doppler shift of the free wave and the breaking wave can be expressed as

[0088] f j =2(v Rf ) / λ=2(V p +v ro ±Vsinθ') / λ

[0089] Where λ is the radar wavelength, and for free waves, V = v f and θ' = θ, while for breaking waves, V = v s and θ'=θ s ; where θ and θ sThey are the local incident angles of the free wave and the breaking wave respectively, where Δθ caused by the movement of the ship must be taken into account, and θ can be expressed as

[0090] θ = θ0 ± Δθ - φ

[0091] The average slope θ of the breaking wave s is

[0092] θ s = θ o ± Δθ - (φ s + φ)

[0093] where φ s represents the average slope of the breaking wave on the mesoscale wave expressed as, less than 20° during simulation. See Step 5 Figure 4 Schematic diagram of the scattering model of the ship platform coupled with free waves and breaking waves.

[0094] 6) Verify the correctness of the simulation model by comparing it with the measured Doppler spectrum of the shipborne coherent microwave radar.

[0095] Verifying the correctness of the simulation model by comparing it with the measured Doppler spectrum of the shipborne coherent microwave radar described in Step 6 mainly compares: the Doppler spectra when the angle between the radar look direction and the wave direction is different; the Doppler spectra when the ship's forward speed is different and the angle between the radar look direction and the ship's bow direction is different; the wave number-frequency spectrum.

[0096] See Figure 5 The simulation and measured comparison diagram of the range Doppler spectrum and the average Doppler spectrum varying with the angle between the radar look direction and the main wave direction. Figure 5 a(1) - g(1) in are the range Doppler spectra of simulations with different angles between the radar look direction and the main wave direction and an effective wave height of 3 m; Figure 5 a(3) - g(3) in are the measured radar range Doppler spectra under the same conditions as a(1) - g(1), where ABRW (the azimuthal angle between the radar look direction and the dominant wave) represents the angle between the radar look direction and the main wave direction. Figure 5 a(2) - g(2) in are the average Doppler spectra of a(1) - g(1); a(4) - g(4) are the average Doppler spectra of a(3) - g(3), and BW (bandwidth) represents the bandwidth. Figure 5 The "A" in a(1) and Figure 5 The peak at the "B" mark in a(3) is generated by breaking. When 90° < ABRW ≤ 180°, in the range Doppler spectrum, the "peak" appears on the right side of the sea surface echo, while when 0° ≤ ABRW < 90°, the "peak" appears on the left side. The average Doppler spectrum except at Figure 5 The sea surface echoes at d(2) and 5d(4) (i.e., ABRW = 88°, with the antenna almost perpendicular to the main wave direction) are symmetrical in shape. The rest of the sea surface echoes show a broadening phenomenon (e.g., the elliptical marks in a(2) and a(4)). This broadening phenomenon is caused by breaking waves. Under the same sea conditions, the average Doppler spectrum widens as the ABRW approaches 0° or 180°. Breaking waves cause "spikes" and broadening. The Doppler spectrum simulated by this model is similar to the Doppler spectrum measured by the radar, and the variation pattern is consistent.

[0097] See also Figure 6 A comparison of the simulated and measured average Doppler spectrum bandwidth as the angle between the radar line of sight and the main wave direction changes. The average Doppler spectrum bandwidth increases as the angle between the radar line of sight and the main wave direction approaches 0° or 180°. The simulated and measured Doppler spectrum bandwidth changes in a consistent pattern.

[0098] See also Figure 7 Comparison diagram of the range Doppler spectrum and average Doppler spectrum at different ship speeds and different angles between the radar line of sight and the ship's bow. Figure 7 The range Doppler spectra of a(1)~i(1) simulated at different ship speeds and different angles between radar sighting direction and ship bow direction; Figure 7 a(3)~i(3) are the radar range Doppler spectra under the same conditions as a(1)~i(1), where ABRF (the azimuthal angle between the radar look direction and the forward direction of ship) represents the angle between the radar look direction and the bow direction of the ship, and v F Indicates ship speed. Figure 7 a(2)~i(2) is the average Doppler spectrum of a(1)~i(1); a(4)~i(4) is the average Doppler spectrum of a(3)~i(3). F =0knot(when the ship stops sailing) Figure 7 As shown in a~c. Figure 7 The simulated average Doppler spectra of a(2), b(2), and c(2) have the same bandwidth of 37.3 Hz at ABRF of 15°, 45°, and 75°; Figure 7 The average Doppler spectrum of a(4), b(4), and c(4) measured at ABRF of 15°, 45°, and 75° also has the same bandwidth of 37.3 Hz. Obviously, ABRF does not produce Doppler spectrum expansion. In contrast, when the ship is sailing, for example, at v F=12knot, such as g(2), h(2), and i(2), the simulated average Doppler spectrum has bandwidths of 43.2Hz, 62.9Hz, and 70.7Hz at ABRF of 15°, 45°, and 75°, respectively; such as g(4), h(4), and i(4), the average Doppler spectrum measured by radar has bandwidths of 47.1Hz, 62.9Hz, and 80.5Hz at ABRF of 15°, 45°, and 75°, respectively. The simulated Doppler spectrum is in good agreement with the measured data, and the Doppler spectrum bandwidth is increased during the ship's navigation. Such as c(2), f(2), and i(2), the simulated average Doppler spectrum v F At 0 knot, 8 knot, and 12 knot, the bandwidth is 37.3 Hz, 62.9 Hz, and 70.7 Hz respectively; Figure 7 The c(4), f(4), and i(4) measured average Doppler spectrum at ABRF = 75° and v F For 0 knot, 8 knot, and 12 knot, the bandwidth is 37.3 Hz, 60.9 Hz, and 80.5 Hz respectively. Therefore, the bandwidth of the Doppler spectrum increases with the forward ship speed v. F Comparing d~f and g~i, the Doppler shift increases as the ABRF decreases when the ship is sailing. The above characteristics and trends obtained by the model are consistent with the radar measured spectrum.

[0099] See also Figure 8 A comparison of the average spectral width of the average Doppler spectrum of a ship is shown in simulation and measured data, as the angle between the radar line of sight and the ship's bow changes. As the ship sails, the average bandwidth of the average Doppler spectrum increases as the angle between the radar line of sight and the ship's bow decreases. The average bandwidth also increases with increasing forward speed. The simulation results generally agree with the measured data on the bandwidth variation pattern.

[0100] See also Figure 9 The comparison between the simulated and measured wavenumber-frequency spectrum is shown in the figure. In the comparison figure, the ship speed is 10 knots, the effective wave height is 3m, and the angle between the radar line of sight and the bow direction is 15°. Figure 9 (a) and (d) (the angle between the radar line of sight and the main wave direction is 180°), Figure 9 (b) and (e) (the angle between the radar line of sight and the main wave direction is 90°), Figure 9 The energy distribution of (c) and (f) (the angle between the radar line of sight and the main wave direction is 0°) verifies the correctness of the simulation results.

[0101] The echo modeling method for shipborne coherent microwave oceanographic radars with fragmentation coupling, developed in this paper, addresses the existing lack of a systematic explanation of the scattering mechanism and echo modeling simulation for shipborne coherent microwave radars. This method takes into account factors such as the ship's forward motion, the ship's six-degree-of-freedom roll, the angle between the radar illumination direction and the ship's bow, and breaking waves. This method is beneficial for improving the wave measurement performance of shipborne coherent microwave radars and promoting their widespread application.

[0102] The main content of the present invention is to establish a model to explain some characteristic changes of microwave radar on ship-borne platforms compared with shore-based platforms, and to guide the subsequent wave inversion of ship-borne coherent microwave radar.

Claims

1. A method for echo modeling of a shipborne coherent microwave ocean radar, characterized in that: include: Conduct three-dimensional sea surface modeling to simulate the three-dimensional sea surface; According to the conditions of the occurrence of three-dimensional sea surface breaking waves, the instantaneous state of the breaking waves and the evolution process of the breaking waves are simulated: the breaking waves are generated when the wave displacement slope is less than -tan7°, the duration of the breaking wave evolution process is 0.8 mesoscale wave cycles, and the phase velocity of the mesoscale wave is Calculated as: Where, is the phase velocity of the primary wave; A ship motion model based on the 3rd harmonic statistical model of periodic motion is simulated. By changing the amplitude and angular frequency of each harmonic of the ship motion model, the ship motion model under different sea conditions is simulated to simulate the ship swaying state under different sea conditions. The calculation expression of the swaying angle of the ship motion model is: Hengyao: Long distance: Bow remote: Where, 、 、 are the amplitudes of the harmonics of roll, pitch and yaw respectively; 、 、 are the angular frequencies of the harmonics of roll, pitch and yaw respectively; Simulation of composite sea surface echo velocity model modulated by ship motion; Using the small amplitude scattering theory, the scattering mechanism of free waves and breaking waves of coherent microwave ocean radar under the ship grazing angle is modeled, and the Doppler spectrum simulation model of sea surface echo of coherent microwave ocean radar is obtained.

2. The method according to claim 1, characterized in that The composite sea surface echo velocity model modulated by ship motion is the velocity contribution caused by the ship's forward motion and attitude, and is expressed by the radial velocity of free wave backscattering as: The radial velocity backscattered by the breaking wave is expressed as: Where, is the phase velocity of the free wave; represents the speed of the breaking wave during the breaking evolution process, that is, First, it is equal to the mesoscale wave phase velocity and the azimuth between the radar line of sight and the main wave direction The product of , eventually reduced to the phase velocity of the free wave ; The positive and negative signs indicate that the Bragg wave is moving towards and away from the antenna, respectively; is the radial orbital velocity of the long wave, which is the main periodic modulation term of the Doppler velocity; is the radar grazing angle; is the radial velocity caused by the ship's forward motion and attitude, expressed as: Where, It is the angle between the radar line of sight and the ship's heading; are the velocity components in the three directions of the coordinate system generated by the ship's motion and attitude; Indicates three linear speeds: horizontal and vertical linear speed , longitudinal and remote linear speed and bow telescopic linear speed , the modeling is calculated by combining the position of the antenna relative to the center of gravity of the ship and the swing angle of the ship motion model, Use the following formula conversion for modeling: Where, , and They are roll speed, pitch speed, and heave speed respectively; Indicates the forward speed of the ship.

3. The method according to claim 2, characterized in that The simulation model of the Doppler spectrum of the sea surface echo of the coherent microwave ocean radar is expressed as: In the formula The Doppler spectrum obtained by free wave scattering The Doppler spectrum obtained by breaking wave scattering In the above two formulas, is the radial distance, It's time, is the beamwidth, Electromagnetic wave number, Doppler frequency, is the geometric factor for vertical polarization, Shortwave spectral density, represents the long-wave tilt angle of the incident plane, is the probability distribution of the long wave slope, and The shape of the Doppler spectrum follows a Gaussian distribution: Where, for or ; The width of the Doppler spectrum, caused by free waves; and It is determined by the free wave and the limited range of the breaking wave. The Doppler shift of the free wave and the breaking wave is expressed as Where, is the radar wavelength; for free waves, and ; and for breaking waves, and ;in and are the local incident angles of free waves and breaking waves, respectively, where the motion of the ship causes To be taken into consideration, Expressed as Average slope of breaking waves for in Expressed as the average slope of breaking waves on mesoscale waves.

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